TOKYO (Reuters) - Hatsune Miku has a following that would make most Japanese pop stars green with envy, with thousands of fans at every concert and a big international following.

She never misses a beat, fluffs a line or messes up a step. But then she doesn't really exist.

Hatsune Miku is computer generated, based on a voice-synthesizing programme developed by the company Crypton Future Media that allows users to create their own music.

Her image was produced by the company, but her music is a creation of her fans, Her best songs -- the ones headlined at her concerts -- have emerged from more than 20 different people.

The fans know what the fans like.

All 10,000 tickets for the digital diva's four shows in Tokyo -- two on Thursday and two on Friday -- sold out in hours despite the 6,300 yen ($76) ticket price.

Hatsune Miku was projected onto the stage at the shows while thousands of other fans packed into 24 cinemas to watch live.

"It was absolutely amazing, it's like my heart is still dancing. I don't think I'll be able to sleep," 21-year-old Yuya Ofuji said as she came out of a concert.

Another fan, Hazuki Koide, showed her dedication by dressing up as Hatsune Miku.

"I've liked her for a long time and wasn't able to come to the concert last year and watched it in a movie theatre. But this year I thought that I absolutely had to make it," Koide said.

The concert, billed as possibly Hatsune Miku's last, was also broadcast in cinemas in Shanghai, Hong Kong and Taiwan.

Some fans came from further afield to catch what could be their idol's last gig.

"We thought we really had to make a real effort to come because we wouldn't get a chance to see her in the future," said Daniel Noll who flew in from Australia.

It's not clear why organizers said these shows could be Hatsune Miku's last, but if they are, she'll be going out on a high. Some online polls have her down as the most-requested singer for the London Olympics opening ceremony.

Whatever her future, the virtual star has made a real difference to many fans, they say.

"She gave a lot of people that didn't have a voice, a voice to express their feelings and thoughts," Noll said.